Summary: Recent study indicates that telomerase activity is highly regulated in human lymphocyte development, differentiation, and activation. However, the mode of telomerase regulation and the functional importance of telomerase in lymphocyte function remain unclear. The first part of this project is to determine the regulation of telomerase catalytic subunit, hTERT, expression and its relationship with telomerase activity in human lymphocytes. We found that that telomerase activity in lymphocytes can be controlled by mechanisms other than transcriptional or translational regulation of hTERT. Moreover, we have demonstrated that induction of telomerase activity in activated T cells is controlled by PI-3 kinase-mediated signaling events and that hTERT is phosphorylated and translocated from cytoplasm to nucleus in response to CD4+ T cell activation. Thus, telomerase activity in human lymphocytes is regulated by novel mechanisms distinct from transcriptional or translational regulation of hTERT expression. To determine how telomerase activity is regulated in lymphocytes, we are currently analyzing hTERT interaction with other proteins in human lymphocytes. The second part of this project is to investigate age effects on telomerase expression and telomere length in human lymphocyte subsets. We have analyzed lymphocyte subsets from 121 normal individuals aged from newborn to 94 years and revealed several new findings: 1) Telomere shortening was observed in CD4+ and CD8+ T cells and B cells with age. However, the rate of telomere loss was significantly different in these populations, 35 +/- 8, 26 +/- 7, and 19 +/- 7 bp/year for CD4+, CD8+ T and B cells, respectively. In addition, CD4+ T cells had the longest average telomeres at all ages, followed by B cells, with CD8+ T cell telomeres the shortest, suggesting that these lymphocyte populations may have different replicative histories in vivo. 2) Telomerase activity in freshly isolated T and B cells was indistinguishably low to undetectable at all ages, but was markedly increased after antigen- and costimulatory receptors mediated stimulation in vitro. Furthermore, age did not alter the magnitude of telomerase activity induced after stimulation of T or B lymphocytes through antigen and costimulatory receptors or in response to PMA plus ionomycin treatment. 3) The levels of telomerase activity induced by in vitro stimulation varied among individual donors but were highly correlated with the outcome of telomere length change in CD4+ T cells after antigen receptor mediated activation. Together, these results indicate that rates of age-associated loss of telomere length in vivo in peripheral blood lymphocytes is specific to T and B cell subsets and that age does not significantly alter the capacity for telomerase induction in lymphocytes.